This invention relates to the improvement of the quality of a multiple coated cellulosic sheet material which is prepared by applying, to a cellulosic sheet material base, two or more layers of a mineral pigment-containing coating composition to form a bonded unitary structure having suitable surface properties.
If a high quality printed image is to be applied to a cellulosic sheet material such as paper or cardboard it is generally necessary to apply to the surface at least one coating composition layer containing one or more mineral pigments such as kaolin clay, calcium carbonate, calcium sulphate, titanium dioxide, barium sulphate, satin white and the like. The application of such a coating composition layer improves the smoothness, gloss, whiteness and opacity of the surface to which the printed image is to be applied. In many cases, in order to obtain a final coated surface of the desired quality, it is necessary to apply two or more layers of pigment-containing coating compositions. For example, a first coating composition layer may be employed to smooth the profile roughness of and cover voids present in the base cellulosic layer and a second coating composition may be employed to adhere to and coat the first layer and to give a better quality surface finish than the first layer and/or complete the pigment coverage or `hiding` of the underlying base provided by the first layer especially where that is relatively dark, eg. comprising board. The materials of the two or more coating composition layers may be the same or different. Generally, they are different. The pigment material of the outer layer is usually finer and more expensive than that of the first coating composition applied. In any event, the resulting product eventually comprises multiple inseparable layers bonded together as a unitary structure providing a high quality surface on at least one side to which a printed image may be applied.
Paper coating composition layers are applied in a well known way using coating machinery, eg. in which the coating composition is applied to the underlying layer by a so called `doctor blade`. Generally, in order to receive the maximum return from capital invested in paper coating machinery, it is desirable to run the coating machine at the highest practicable web speed. Also, since the coating compositions consist of pigment, adhesives, and possibly other solid ingredients in suspension in water, it is necessary to remove the water content of the composition by thermal evaporation in order to dry the coatings. In order to minimize the consumption of energy for thermal evaporation it is desirable to operate with coating compositions having the highest possible solids concentrations. However, it is found that when a final coating composition of relatively high solids concentration is applied to a base sheet of relatively high water absorbency, there is a tendency, when a doctor blade is used to remove excess coating composition and smooth the coating, for this final coating to be marred by scratches and other defects.
Where such scratches and defects occur it can be very costly to the manufacturer, since the machinery will need to be stopped sometimes for considerable periods of time, to correct the fault. The general ability to use a paper coating composition continuously in a paper coating operation without difficulty is known in the art as "runnability". Another problem which occurs in the manufacture of multiple coated paper and like products is to obtain good adhesion of an outer coating composition layer to an underlying coating composition layer on a cellulosic sheet. This is because the underlying coating composition layer will usually have a smoothness which is much improved compared with the underlying cellulosic sheet and it is difficult to ensure that the outer layer keys onto the surface of the smoother underlying layer.
An object of the present invention is to provide a method of depositing multiple layers of pigment-containing coating compositions on a base cellulosic sheet in a manner in which the runnability of the product may be improved without significantly interfering with the adhesion between the coating layers.